Control apparatus for internal combustion engine

ABSTRACT

A control apparatus for an internal combustion engine for adjusting an amount of air passing through a catalyst during fuel cut-off operation of the internal combustion engine, the temperature of the catalyst is caused to rise, when the fuel cut-off operation of the internal combustion engine is carried out in a state where the temperature of the catalyst is low. The apparatus is constructed such that in cases where the fuel cut-off operation is carried out in a state where the temperature of the catalyst is relatively low but equal to or higher than an activation temperature thereof, the amount of air passing through the catalyst is made larger in a period of time in which the catalyst becomes a rich atmosphere immediately after the start of the fuel cut-off operation, in comparison with a subsequent period of time in which the catalyst becomes a lean atmosphere.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a National Stage of International Application No.PCT/JP2015/004317, filed on Aug. 27, 2015, which claims priority fromJapanese Patent Application No. 2014-196813, filed on Sep. 26, 2014, thecontents of all of which are incorporated herein by reference in theirentirety.

TECHNICAL FIELD

The present invention relates to a control apparatus for an internalcombustion engine, and in particular, to a technology to adjust anamount of air passing through a catalyst during fuel cut-off operationof the internal combustion engine.

BACKGROUND ART

There is known a technology in which in an arrangement where a catalystis disposed in an exhaust passage of an internal combustion engine, whenthe internal combustion engine is driven in a fuel cut-off operation, anamount of gas (an amount of air) passing through the catalyst is made toincrease, thereby suppressing an excessive rise in the temperature ofthe catalyst. In addition, there is also known a technology in which inthe case where the fuel cut-off operation of the internal combustionengine is carried out, when the temperature of the catalyst is low, theamount of air passing through the catalyst is made to decrease, therebysuppressing excessive cooling of the catalyst (for example, see a firstpatent literature).

CITATION LIST Patent Literature

Patent Literature 1: Japanese patent laid-open publication No.2004-132185

Patent Literature 2: Japanese patent laid-open publication No.2002-371836

SUMMARY OF INVENTION Technical Problem

However, in the case where the fuel cut-off operation of the internalcombustion engine is carried out in a state where the temperature of thecatalyst is low, even if the amount of air passing through the catalystis decreased, it may become difficult to maintain the catalyst in anactivated state.

The present invention has been made in view of the actual circumstancesas referred to above, and has for its object to provide a technology inwhich in a control apparatus for an internal combustion engine whichserves to adjust an amount of air passing through a catalyst during fuelcut-off operation of the internal combustion engine, the catalyst can bemaintained in an activated state, when the fuel cut-off operation of theinternal combustion engine is carried out in a state where thetemperature of the catalyst is low.

Solution to Problem

In solving the above-mentioned problem, the inventors of the presentinvention have focused on the following fact: in the case where acatalyst having an oxygen storage ability is disposed in an exhaustpassage of an internal combustion engine, when fuel cut-off operation ofthe internal combustion engine is carried out, the catalyst becomes anatmosphere in which the air fuel ratio is equal to or less than astoichiometric air fuel ratio (hereinafter, referred to as a “richatmosphere”), immediately after the start of the fuel cut-off operation,and thereafter becomes an atmosphere in which the air fuel ratio ishigher than the stoichiometric air fuel ratio (hereinafter, referred toas a “lean atmosphere”). Then, in cases where the fuel cut-off operationis carried out in a state where the temperature of the catalyst is in arelatively low temperature range which is equal to or higher than anactivation temperature thereof, the amount of air passing through thecatalyst is made to be larger in a period of time in which the catalystbecomes a rich atmosphere immediately after the start of the fuelcut-off operation, in comparison with a subsequent period of time inwhich the catalyst becomes a lean atmosphere.

Specifically, according to the present invention, there is provided acontrol apparatus for an internal combustion engine in which in anexhaust passage, there is disposed a catalyst having an oxygen storageability in which oxygen is stored when an air fuel ratio of exhaust gasis a lean air fuel ratio higher than a stoichiometric air fuel ratio,whereas oxygen is released when the air fuel ratio of exhaust gas is arich air fuel ratio lower than the stoichiometric air fuel ratio, saidapparatus comprising:

an estimation unit configured to estimate the air fuel ratio of theexhaust gas in said catalyst;

an obtaining unit configured to obtain a temperature of said catalyst;

an adjustment unit configured to adjust an amount of air passing throughsaid catalyst to a predetermined cooling suppression amount, when thetemperature obtained by said obtaining unit is equal to or less than apredetermined low-temperature determination temperature which is higherthan an activation temperature of said catalyst, in a period of time inwhich the internal combustion engine is driven in a fuel cut-offoperation; and

a correction unit configured to increase the amount of air passingthrough said catalyst from said cooling suppression amount, when thetemperature obtained by said obtaining unit is equal to or higher thanthe activation temperature of said catalyst, during a period of timefrom a point in time at which the fuel cut-off operation is starteduntil a point in time at which the air fuel ratio estimated by saidestimation unit changes from an air fuel ratio equal to or less than thestoichiometric air fuel ratio to a lean air fuel ratio, in the casewhere the amount of air passing through said catalyst is adjusted tosaid cooling suppression amount by said adjustment unit.

The “low-temperature determination temperature” referred to herein is atemperature at which in cases where the fuel cut-off operation of theinternal combustion engine is carried out at the time where thetemperature of the catalyst is equal to or lower than saidlow-temperature determination temperature, when the low-load operationof the internal combustion engine is carried out during the fuel cut-offoperation or after the end of the fuel cut-off operation, it can beassumed that the temperature of the catalyst may drop to less than theactivation temperature. This low-temperature determination temperaturehas been beforehand obtained experimentally. In addition, the “coolingsuppression amount” is an amount which is set in such a manner that theamount of heat carried away from the catalyst with the air passingthrough the catalyst becomes a minimum, and it has been beforehanddecided by adaptation work making use of experiments, etc.

While the fuel cut-off operation of the internal combustion engine iscarried out, the air sucked into a cylinder is discharged out of thecylinder, without being supplied for combustion. For that reason, thegas flowing into the catalyst during a period of time of the fuelcut-off operation contains only air. However, immediately after thestart of the fuel cut-off operation, a mixed gas containing air and gas(burnt gas) combusted in the cylinder immediately before the start ofthe fuel cut-off operation flows into the catalyst. In that case, oxygenin the mixed gas is stored in the catalyst, so that the atmosphere ofthe catalyst becomes a rich atmosphere (i.e., the air fuel ratio of theexhaust gas inside the catalyst is the stoichiometric air fuel ratio ora rich air fuel ratio). Then, when the oxygen storage ability of thecatalyst is saturated, the atmosphere of the catalyst changes from therich atmosphere to a lean atmosphere (i.e., the air fuel ratio of theexhaust gas inside the catalyst changes from the air fuel ratio equal toor less than the stoichiometric air fuel ratio to a lean air fuelratio).

Here, in the case where the catalyst immediately after the start of thefuel cut-off operation is exposed to the rich atmosphere, when thecatalyst has been activated, hydrocarbon (HC) and carbon monoxide (CO)contained in the mixed gas are made to react with oxygen (O₂) under theaction of the catalyst (oxidation reaction), so that the catalyst isheated by the heat of reaction generated. On the other hand, after theatmosphere of the catalyst has changed from the rich atmosphere to thelean atmosphere, the oxidation reaction as mentioned above will notsubstantially occur, and the heat of the catalyst will be carried awaywith the air passing through the catalyst.

Accordingly, in the case where the amount of air passing through saidcatalyst is adjusted to the cooling suppression amount by means of theadjustment unit, when the temperature of the catalyst obtained by theobtaining unit is equal to or higher than the activation temperaturethereof during the period of time (hereinafter, referred to as “a richperiod of time”) from the point in time at which the fuel cut-offoperation is started until the point in time at which the air fuel ratioestimated by the estimation unit changes from the air fuel ratio equalto or less than the stoichiometric air fuel ratio to the lean air fuelratio, and when the amount of air passing through the catalyst is mademore than the cooling suppression amount, the amount of oxygen flowinginto the catalyst per unit time increases. As the amount of oxygenflowing into the catalyst per unit time increases, the amount ofoxidation reaction heat generated per unit time also increases. As aresult, the heating of the catalyst by the heat of oxidation reactionbecomes prevalent with respect to the carrying away of heat by the airpassing through the catalyst, so that the amount of rise in thetemperature of the catalyst during the rich period of time increases. Inaddition, after the lapse of the rich period of time, the amount of airpassing through the catalyst is decreased to the predetermined coolingsuppression amount, so the amount of heat carried away from the catalystwith the air passing through the catalyst is suppressed to a minimum,thereby decreasing the amount of drop in the temperature of thecatalyst.

Accordingly, according to the control apparatus for an internalcombustion engine of the present invention, in cases where the fuelcut-off operation of the internal combustion engine is carried out inthe state where the temperature of the catalyst is low, it becomesdifficult for the temperature of the catalyst to drop to less than theactivation temperature at the time of the low-load operation during thefuel cut-off operation or after the end of the fuel cut-off operation.

Here, note that when the temperature of the catalyst is higher than saidlow-temperature determination temperature, in the period of time inwhich the fuel cut-off operation of the internal combustion engine iscarried out, said adjustment unit may adjust the amount of air passingthrough the catalyst to a predetermined temperature rise suppressionamount. The “temperature rise suppression amount” referred to herein isan amount which is larger than said cooling suppression amount, andwhich is set in such a manner that the amount of heat carried away fromthe catalyst with the air passing through the catalyst becomes as muchas possible, and it has been beforehand obtained by adaptation workmaking use of experiments, etc.

However, when the amount of air passing through the catalyst is adjustedto the temperature rise suppression amount in the case where the fuelcut-off operation of the internal combustion engine is carried out in astate where the temperature of the catalyst is sufficiently higher thansaid low-temperature determination temperature, the amount of rise inthe temperature of the catalyst in said rich period of time increases,so the temperature of the catalyst may rise to an excessive extentduring the fuel cut-off operation.

In contrast to this, the correction unit of the present invention maydecrease the amount of air passing through said catalyst to a valuesmaller than said temperature rise suppression amount, when thetemperature of the catalyst obtained by said obtaining unit is equal toor higher than a predetermined high-temperature determinationtemperature, during the period of time (the rich period of time) fromthe point in time at which the fuel cut-off operation is started untilthe point in time at which the air fuel ratio estimated by saidestimation unit changes from the air fuel ratio equal to or less thanthe stoichiometric air fuel ratio to the lean air fuel ratio, in thecase where the amount of air passing through said catalyst is adjustedto said temperature rise suppression amount by said adjustment unit. The“high-temperature determination temperature” referred to herein is atemperature which is sufficiently high as compared with saidlow-temperature determination temperature, and which can be assumed thatthe temperature of the catalyst may rise to an excessive extent duringthe fuel cut-off operation, when the fuel cut-off operation of theinternal combustion engine is carried out in a state where thetemperature of the catalyst is higher than said high-temperaturedetermination temperature. This high-temperature determinationtemperature has been beforehand obtained experimentally.

According to such a configuration, in cases where the fuel cut-offoperation of the internal combustion engine is carried out in a statewhere the temperature of the catalyst is sufficiently high, as comparedwith the low-temperature determination temperature, it is possible tomake slow the rate of oxidation reaction during the rich period of time.For that reason, the amount of rise in the temperature of the catalystin the rich period of time can be suppressed small, so that thetemperature of the catalyst becomes difficult to rise to an excessiveextent during the fuel cut-off operation of the internal combustionengine.

Here, in cases where a measuring device (e.g., an oxygen concentrationsensor or an air fuel ratio sensor) for measuring a physical quantity(e.g., an oxygen concentration or an air fuel ratio) correlated with theair fuel ratio of the exhaust gas is disposed in the exhaust passage atthe downstream side of the catalyst, the estimation unit may estimatethe air fuel ratio of the exhaust gas (atmosphere) inside the catalyst,by assuming that the air fuel ratio measured by the measuring device isequal to the air fuel ratio of the exhaust gas inside the catalyst.

In addition, the estimation unit may estimate the air fuel ratio of theexhaust gas inside the catalyst, by using as parameters an amount ofoxygen flowing into the catalyst after the start of the fuel cut-offoperation (a multiplied value of the amount of intake air and theconcentration of the oxygen contained in the air) and an oxygen storagecapacity of the catalyst. For example, the amount of oxygen flowing intothe catalyst may be integrated from the point in time at which the fuelcut-off operation was started, and in a period of time until theintegrated value thus obtained reaches the oxygen storage capacity ofthe catalyst, it may be estimated that the air fuel ratio of the exhaustgas inside the catalyst is equal to or less than the stoichiometric airfuel ratio.

Advantageous Effects of Invention

According to the present invention, in a control apparatus for aninternal combustion engine which serves to adjust an amount of airpassing through a catalyst during fuel cut-off operation of the internalcombustion engine, it is possible to raise the temperature of thecatalyst in an appropriate and suitable manner, when the fuel cut-offoperation of the internal combustion engine is carried out in a statewhere the temperature of the catalyst is low.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a view showing the schematic construction of an internalcombustion engine with its intake and exhaust systems to which thepresent invention is applied.

FIG. 2 is a timing chart showing a method for adjusting an amount ofintake air in cases where fuel cut-off operation of the internalcombustion engine is carried out in a state where the temperature of acatalyst becomes equal to or lower than a low-temperature determinationtemperature.

FIG. 3 is a flow chart showing a method for adjusting the amount ofintake air in cases where the fuel cut-off operation of the internalcombustion engine is carried out in a state where the temperature of thecatalyst becomes higher than the low-temperature determinationtemperature.

FIG. 4 is a flow chart showing a processing routine which is executed byan ECU in cases where the fuel cut-off operation of the internalcombustion engine is carried out, in a first embodiment of the presentinvention.

FIG. 5 is a timing chart showing a method for adjusting an amount ofintake air in cases where the fuel cut-off operation of the internalcombustion engine is carried out in a state where the temperature of thecatalyst becomes equal to or higher than a high-temperaturedetermination temperature.

FIG. 6 is a flow chart showing a processing routine which is executed byan ECU in cases where the fuel cut-off operation of the internalcombustion engine is carried out, in a second embodiment of the presentinvention.

DESCRIPTION OF EMBODIMENTS

Hereinafter, specific embodiments of the present invention will bedescribed based on the attached drawings. However, the dimensions,materials, shapes, relative arrangements and so on of component partsdescribed in the embodiments are not intended to limit the technicalscope of the present invention to these alone in particular as long asthere are no specific statements.

Embodiment 1

First, reference will be made to a first embodiment of the presentinvention based on FIGS. 1 through 4. FIG. 1 is a view showing theschematic construction of an internal combustion engine 1 and its intakeand exhaust systems, to which the present invention is applied. Theinternal combustion engine 1 shown in FIG. 1 is a spark ignition typeinternal combustion engine (gasoline engine) having a plurality ofcylinders. Here, note that in FIG. 1, only one cylinder among theplurality of cylinders is illustrated.

A piston 3 is fitted in each cylinder 2 of the internal combustionengine 1 for sliding movement relative thereto. The piston 3 isconnected with an unillustrated engine output shaft (crankshaft) througha connecting rod 4. On each cylinder 2, there are mounted a fuelinjection valve 5 for injecting fuel into the cylinder 2 and a sparkplug 6 for generating a spark in the cylinder 2.

The inside of each cylinder 2 is in communication with an intake port 7and an exhaust port 8. An open end of the intake port 7 in each cylinder2 is opened and closed by means of an intake valve 9. An open end of theexhaust port 8 in each cylinder 2 is opened and closed by means of anexhaust valve 10. The intake valve 9 and the exhaust valve 10 are drivento open and close by means of an unillustrated intake cam and anunillustrated exhaust cam, respectively.

The intake port 7 is in communication with an intake passage 70. Athrottle valve 71 is arranged in the intake passage 70. An air flowmeter 72 is arranged in the intake passage 70 at a location upstream ofthe throttle valve 71.

The exhaust port 8 is in communication with an exhaust passage 80. Anexhaust gas purification device 81 is arranged in the exhaust passage80. The exhaust gas purification device 81 receives at least a catalysthaving an oxidation function (e.g., a three-way catalyst, an NOx storagereduction catalyst, an oxidation catalyst, or the like) in a cylindricalcasing. Here, note that the catalyst received in the exhaust gaspurification device 81 has an oxygen storage ability which stores oxygenwhen the air fuel ratio of the exhaust gas is a lean air fuel ratiohigher than a stoichiometric air fuel ratio, and releases oxygen whenthe fuel ratio of the exhaust gas is a rich air fuel ratio lower thanthe stoichiometric air fuel ratio.

In the internal combustion engine 1 constructed in this manner, there isarranged in combination therewith an ECU (Electronic Control Unit) 20for controlling the internal combustion engine 1. The ECU 20 is anelectronic control unit which is composed of a CPU, a ROM, a RAM, abackup RAM, and so on. Detection signals of various sensors, such as awater temperature sensor 11, a crank position sensor 21, an acceleratorposition sensor 22, an exhaust gas temperature sensor 82, an air fuelratio sensor (A/F sensor) 83, and so on, in addition to a detectionsignal of the above-mentioned air flow meter 72, are input to the ECU20.

The air flow meter 72 outputs an electrical signal correlated with anamount (mass) of intake air flowing in the intake passage 70. The watertemperature sensor 11 outputs an electrical signal correlated with thetemperature of cooling water circulating in the internal combustionengine 1. The crank position sensor 21 outputs an electrical signalcorrelated with the rotational position of the crankshaft of theinternal combustion engine 1. The accelerator position sensor 22 outputsan electric signal correlated with an amount of operation of anunillustrated accelerator pedal (i.e., a degree of accelerator opening).The exhaust gas temperature sensor 82 is arranged in the exhaust passage80 at a location downstream of the exhaust gas purification device 81,and outputs an electrical signal correlated with the temperature of theexhaust gas flowing out of the exhaust gas purification device 81. Theair fuel ratio sensor 83 is arranged in the exhaust passage 80 at alocation downstream of the exhaust gas purification device 81, andoutputs an electrical signal correlated with the air fuel ratio of theexhaust gas flowing out of the exhaust gas purification device 81.

The ECU 20 is electrically connected to a variety of kinds of equipmentsuch as the fuel injection valve 5, the spark plug 6, the throttle valve71, etc., and controls the variety of kinds of equipment based on theoutput signals of the above-mentioned variety of kinds of sensors. Forexample, the ECU 20 carries out known control, such as fuel injectioncontrol, etc., according to an operating state of the internalcombustion engine 1 specified by the output signals of the crankposition sensor 21, the accelerator position sensor 22, the air flowmeter 72, and so on. In addition, the ECU 20 carries out air amountadjustment processing to adjust the amount of air passing through theexhaust gas purification device 81 during the fuel cut-off operation ofthe internal combustion engine 1. In the following, a method of carryingout the air amount adjustment processing in this embodiment will bedescribed.

When the engine rotation speed calculated from the output signal of thecrank position sensor 21 is equal to or more than a predetermined enginerotation speed, and when the accelerator opening degree specified fromthe output signal of the accelerator position sensor 22 is zero, the ECU20 causes the internal combustion engine 1 to perform the fuel cut-offoperation by stopping the operation (fuel injection) of the fuelinjection valves 5 in all the cylinders 2. When the accelerator openingdegree becomes large than zero, or when the engine rotation speedbecomes equal to or less than a predetermined threshold value, such afuel cut-off operation is ended, and the operation of the fuel injectionvalves 5 is resumed.

In addition, during the fuel cut-off operation of the internalcombustion engine 1, the ECU 20 controls the throttle valve 71 in such amanner that the amount of intake air in the internal combustion engine 1becomes equal to or more than a predetermined lower limit value, and atthe same time, equal to or less than a predetermined upper limit value.The “lower limit value” referred to herein is, for example, a minimumamount of intake air in which an amount of oil sucked into a cylinder 2due to an increase of negative pressure (a decrease of pressure) in thecylinder accompanying a decrease in the amount of intake air falls belowa permissible amount. On the other hand, the “upper limit value” is, forexample, a maximum amount of intake air in which a sufficient negativepressure can be supplied to an unillustrated brake booster from theintake passage 70 downstream of the throttle valve 71, or a maximumamount of intake air in which engine brake can be secured.

Specifically, when the temperature of the catalyst received in theexhaust gas purification device 81 is equal to or lower than apredetermined low-temperature determination temperature in the period oftime in which the fuel cut-off operation of the internal combustionengine 1 is carried out, the ECU 20 controls the throttle valve 71 insuch a manner that the amount of intake air in the internal combustionengine 1 becomes a predetermined cooling suppression amount.

The “low-temperature determination temperature” referred to herein is atemperature which is higher than an activation temperature (e.g., 400degrees C.) of the catalyst, and which is considered that in cases wherethe fuel cut-off operation of the internal combustion engine 1 iscarried out in a state where the temperature of the catalyst is lowerthan the low-temperature determination temperature, and in cases wherethe amount of intake air during the period of time of the fuel cut-offoperation is made more than the cooling suppression amount, when thelow-load operation of the internal combustion engine 1 is carried outduring the fuel cut-off operation or after the end of the fuel cut-offoperation, the temperature of the catalyst in the exhaust gaspurification device 81 may drop to less than the activation temperaturethereof. This low-temperature determination temperature is a temperature(e.g., about 600 degrees C.) which has been obtained experimentally inadvance.

In addition, the “cooling suppression amount” is an amount which isdecided in such a manner that the amount of heat carried away from thecatalyst with the air passing through the exhaust gas purificationdevice 81 during the fuel cut-off operation of the internal combustionengine 1 becomes as small as possible, and which may be set, forexample, to said lower limit value (the minimum amount of intake air inwhich the amount of oil sucked into a cylinder 2 due to an increase ofnegative pressure in the cylinder 2 accompanying a decrease in theamount of intake air falls below the permissible amount), or an amountequal to a sum of the lower limit value and a predetermined margin.

During the fuel cut-off operation of the internal combustion engine 1,the air sucked into a cylinder 2 is discharged out of the cylinder 2,without being supplied for combustion. For that reason, the gas flowinginto the exhaust gas purification device 81 during the period of time ofthe fuel cut-off operation is a gas of low temperature and leanatmosphere containing only air. Accordingly, when the amount of intakeair during the period of time of the fuel cut-off operation is limitedto said cooling suppression amount, the amount of heat, which is carriedaway from the catalyst by the air passing through the exhaust gaspurification device 81, can be suppressed to a minimum.

However, immediately after the fuel cut-off operation has been started,a mixed gas containing air and gas (burnt gas) combusted in the cylinder2 immediately before the fuel cut-off operation is started flows intothe exhaust gas purification device 81. At that time, the oxygen in themixed gas is stored by the catalyst of the exhaust gas purificationdevice 81, so the interior of the exhaust gas purification device 81becomes a rich atmosphere equal to or less than the stoichiometric airfuel ratio. Then, when the oxygen storage ability of the catalyst issaturated, the interior of the exhaust gas purification device 81 willchange from the rich atmosphere to a lean atmosphere.

Here, when the exhaust gas purification device 81 has been activated inthe case where the catalyst immediately after the start of the fuelcut-off operation is exposed to the rich atmosphere, hydrocarbon (HC)and carbon monoxide (CO) contained in the mixed gas are made to reactwith oxygen (O₂) under the action of the catalyst (oxidation reaction),so that the catalyst is heated by the heat of reaction generated. On theother hand, after the atmosphere of the catalyst has changed from therich atmosphere to the lean atmosphere, the oxidation reaction asmentioned above will not substantially occur, and the heat of thecatalyst will be carried away with the air passing through the exhaustgas purification device 81.

Accordingly, in this embodiment, in cases where the fuel cut-offoperation of the internal combustion engine 1 is carried out in a statewhere the temperature of the catalyst received in the exhaust gaspurification device 81 becomes equal to or less than the low-temperaturedetermination temperature, when the temperature of the catalyst is equalto or higher than the activation temperature thereof, in a period oftime (a rich period of time) from a point in time (t1 in FIG. 2) atwhich the fuel cut-off flag (F/C flag) is turned on until a point intime (t2 in FIG. 2) at which the air fuel ratio of the exhaust gas inthe exhaust gas purification device 81 changes from an air fuel ratioequal to or less than the stoichiometric air fuel ratio to a lean airfuel ratio, as shown in FIG. 2, the throttle valve 71 is controlled soas to make the amount of intake air in the internal combustion engine 1equal to a predetermined reaction promotion amount which is more than anamount of intake air (cooling suppression amount) after the lapse of therich period of time. Then, in a period of time from the end time point(t2 in FIG. 2) of the rich period of time until the F/C flag is turnedoff, the throttle valve 71 is controlled so as to make the amount ofintake air in the internal combustion engine 1 equal to the coolingsuppression amount.

Here, note that the rate of the oxidation reaction occurring in thecatalyst during the rich period of time becomes faster (larger) inaccordance with the increasing amount of oxygen supplied to the catalystper unit time. For that reason, it is preferable that the amount ofintake air during the rich period of time be as much as possible.Accordingly, the “reaction promotion amount” referred to herein may beset to the above-mentioned upper limit value (the maximum amount ofintake air in which a sufficient negative pressure can be supplied tothe unillustrated brake booster from the intake passage 70 downstream ofthe throttle valve 71, or the maximum amount of intake air in whichengine brake can be secured), or an amount which is obtained bysubtracting a margin from the upper limit value.

When the amount of intake air during the fuel cut-off operation isadjusted in this manner, the amount of air flowing into the exhaust gaspurification device 81 during the rich period of time becomes more thanthe cooling suppression amount, thus making it possible to speed up therate of the oxidation reaction occurring in the catalyst during the richperiod of time. As a result, the amount of rise in temperature of thecatalyst in the rich period of time can be increased. In addition, afterthe lapse of the rich period of time, the amount of air flowing into theexhaust gas purification device 81 is decreased to the predeterminedcooling suppression amount, so the amount of heat carried away from thecatalyst with the air passing through the exhaust gas purificationdevice 81 can be suppressed to a minimum, thereby making it possible tosuppress the amount of drop in the temperature of the catalyst to aminimum.

Accordingly, even in cases where the fuel cut-off operation of theinternal combustion engine 1 is carried out in a state where thetemperature of the catalyst is low, the temperature of the catalystbecomes difficult to drop to less than the activation temperaturethereof, at the time of the low-load operation of the internalcombustion engine during the fuel cut-off operation or after the end ofthe fuel cut-off operation, as a result of which the catalyst can bemaintained in an activated state.

Here, note that the F/C flag in FIG. 2 is turned on when the enginerotation speed is equal to or more than the predetermined enginerotation speed and when the accelerator opening degree is zero, whereasit is turned off when the engine rotation speed drops to less than thepredetermined engine rotation speed, or when the accelerator openingdegree becomes larger than zero. Then, it is assumed that when the F/Cflag is on, the operation of the fuel injection valves 5 and the sparkplugs 6 in all the cylinders 2 is stopped by means of the ECU 20.

On the other hand, in cases where the fuel cut-off operation of theinternal combustion engine 1 is carried out in a state where thetemperature of the catalyst received in the exhaust gas purificationdevice 81 becomes higher than the above-mentioned low-temperatureactivation temperature, the throttle valve 71 is controlled so as tomake the amount of intake air in the internal combustion engine 1 equalto a predetermined temperature rise suppression amount (t1 in FIG. 3),when the F/C flag has been turned on, as shown in FIG. 3.

The “temperature rise suppression amount” referred to herein is anamount which is more than the above-mentioned cooling suppressionamount, and which is decided in such a manner that the amount of heatcarried away from the catalyst with the air passing through the exhaustgas purification device 81 becomes as much as possible. For example, thetemperature rise suppression amount may be set to the above-mentionedupper limit value (the maximum amount of intake air in which asufficient negative pressure can be supplied to the unillustrated brakebooster from the intake passage 70 downstream of the throttle valve 71,or the maximum amount of intake air in which engine brake can besecured), or an amount which is obtained by subtracting a predeterminedmargin from the upper limit value.

As long as the temperature of the catalyst becomes higher than thelow-temperature activation temperature, the state where the amount ofintake air in the internal combustion engine 1 is made equal to thetemperature rise suppression amount is continued until the F/C flagchanges from on to off. However, in cases where the temperature of thecatalyst has dropped to a temperature equal to or lower than thelow-temperature determination temperature before the F/C flag changesfrom on to off, the amount of intake air in the internal combustionengine 1 is made to decrease from the above-mentioned temperature risesuppression amount to the cooling suppression amount, at a point in timeat which the temperature of the catalyst becomes equal to or lower thanthe low-temperature determination temperature (t4 in FIG. 3).

In the case where the amount of intake air during the fuel cut-offoperation is adjusted in this manner, when the temperature of thecatalyst is higher than the low-temperature activation temperature, theamount of heat carried away from the catalyst by the air passing throughthe exhaust gas purification device 81 becomes large, and hence, it ispossible to suppress the temperature of the catalyst from rising to anexcessive extent at the time of the high-load operation of the internalcombustion engine 1 during the fuel cut-off operation or after the endof the fuel cut-off operation. In addition, when the temperature of thecatalyst drops to the temperature equal to or lower than thelow-temperature determination temperature in the course of the fuelcut-off operation, the amount of heat carried away from the catalyst bythe air passing through the exhaust gas purification device 81 becomessmall, so the temperature of the catalyst is suppressed from dropping toless than the activation temperature thereof at the time of the low-loadoperation of the internal combustion engine 1 during the fuel cut-offoperation or after the end of the fuel cut-off operation. Accordingly,an excessive temperature rise of the catalyst can be suppressed, whilemaintaining the catalyst in its activated state.

Hereinbelow, reference will be made to a control procedure for theamount of intake air in the case where the fuel cut-off operation of theinternal combustion engine 1 is carried out, in line with FIG. 4. FIG. 4is a flow chart showing a processing routine which is carried out bymeans of the ECU 20 in a repeated manner during the operation of theinternal combustion engine 1 (e.g., in a period of time in which anignition switch has been turned on). It is assumed that this processingroutine has been stored in the ROM of the ECU 20 in advance. Here, it isassumed that in FIG. 4, Xfc indicates the F/C flag, and Tcat indicatesthe temperature of the catalyst received in the exhaust gas purificationdevice 81. In addition, it is also assumed that in FIG. 4, Tlowindicates the low-temperature determination temperature, and Taindicates the activation temperature of the catalyst received in theexhaust gas purification device 81. Moreover, it is assumed that AFcatin FIG. 4 indicates the air fuel ratio of the exhaust gas in the exhaustgas purification device 81.

In the processing routine of FIG. 4, first in the processing of stepS101, the ECU 20 reads in the F/C flag Xfc. Subsequently, in theprocessing of step S102, the ECU 20 determines whether the F/C flag Xfcread in by the processing of step S101 is ON. In cases where a negativedetermination is made in the processing of step S102 (Xfc=OFF), the ECU20 ends the execution of this processing routine. In that case, thethrottle valve 71 is controlled so that the amount of intake air in theinternal combustion engine 1 becomes an amount corresponding to theengine load or the engine rotation speed. On the other hand, in caseswhere an affirmative determination is made in the processing of stepS102 (Xfc=ON), the control routine of the ECU 20 goes to the processingof step S103.

In the processing of step S103, the ECU 20 obtains the temperature Tcatof the catalyst received in the exhaust gas purification device 81.Specifically, the ECU 20 may calculate by estimation the temperature ofthe catalyst Tcat from the operation history of the internal combustionengine 1, or may substitute a measured value of the exhaust gastemperature sensor 82 for the temperature of the catalyst Tcat. An“obtaining unit” according to the present invention is achieved bycarrying out the processing of step S103 by means of the ECU 20.

In the processing of step S104, the ECU 20 obtains the air fuel ratioAFcat of the exhaust gas in the exhaust gas purification device 81.Here, note that the air fuel ratio AFcat of the exhaust gas in theexhaust gas purification device 81 is reflected on the measured value ofthe air fuel ratio sensor 83, so the measured value of the air fuelratio sensor 83 may be used as the air fuel ratio AFcat of the exhaustgas in the exhaust gas purification device 81.

In the processing of step S105, the ECU 20 determines whether thetemperature Tcat of the catalyst obtained in the above-mentionedprocessing of step S103 is equal to or lower than the low-temperaturedetermination temperature Tlow. In cases where an affirmativedetermination is made in the processing of step S105 (Tcat Tlow), theroutine of the ECU 20 goes to the processing of step S106.

In the processing of step S106, the ECU 20 determines whether thetemperature Tcat of the catalyst obtained in the above-mentionedprocessing of step S103 is equal to or higher than the activationtemperature Ta of the catalyst. In cases where an affirmativedetermination is made in the processing of step S106 (Tcat≥Ta), theroutine of the ECU 20 goes to the processing of step S107.

In the processing of step S107, the ECU 20 determines whether the airfuel ratio AFcat obtained in the above-mentioned processing of step S104is equal to or lower than the stoichiometric air fuel ratio AFst. Thatis, the ECU 20 determines whether the interior of the exhaust gaspurification device 81 is in a rich atmosphere. Here, note that in anarrangement in which the air fuel ratio sensor 83 is not disposed in theexhaust passage 80 downstream of the exhaust gas purification device 81,the ECU 20 obtains the amount of oxygen which has been supplied to thecatalyst from the point in time at which the fuel cut-off operation hasbeen started until the current point in time, by multiplying theconcentration of oxygen in the air to the integrated value of the amountof intake air from the start time point of the fuel cut-off operation.Then, the ECU 20 may estimate that the interior of the exhaust gaspurification device 81 is in a rich atmosphere, if the amount of oxygenthus obtained is equal to or smaller than the oxygen storage capacity ofthe catalyst, and that the interior of the exhaust gas purificationdevice 81 is in a lean atmosphere, if the amount of oxygen is largerthan the oxygen storage capacity of the catalyst. An “estimation unit”according to the present invention is achieved by carrying out theprocessing of step S106 by means of the ECU 20.

In cases where an affirmative determination is made in theabove-mentioned processing of step S107 (AFcat≤AFst), the current pointin time belongs to the above-mentioned rich period of time in FIG. 2,and the catalyst has been activated. Accordingly, the routine of the ECU20 goes to the processing of step S108, where the throttle valve 71 iscontrolled in such a manner that the amount of intake air in theinternal combustion engine 1 becomes equal to the reaction promotionamount which is larger than the cooling suppression amount. A“correction unit” according to the present invention is achieved bycarrying out the processing of step S107 by means of the ECU 20.

In cases where an affirmative determination is made in theabove-mentioned processing of step S106 (Tcat<Ta), the catalyst has notbeen activated. For that reason, even if the current point in time isduring the rich period of time, oxidation reaction in the exhaust gaspurification device 81 does not substantially occur, resulting in thatthe heat of the catalyst is carried away by the air passing through theexhaust gas purification device 81. Accordingly, the routine of the ECU20 goes to the processing of step S109, where the throttle valve 71 iscontrolled in such a manner that the amount of intake air in theinternal combustion engine 1 becomes equal to the cooling suppressionamount.

In addition, in cases where a negative determination is made in theabove-mentioned processing of step S107 (AFcat>AFst), at the currentpoint in time, the above-mentioned rich period of time has alreadypassed. For that reason, oxidation reaction in the exhaust gaspurification device 81 does not substantially occur, resulting in thatthe heat of the catalyst is carried away by the air passing through theexhaust gas purification device 81. Accordingly, the routine of the ECU20 goes to the processing of step S109, where the throttle valve 71 iscontrolled in such a manner that the amount of intake air in theinternal combustion engine 1 becomes equal to the cooling suppressionamount, as in the case where a negative determination is made in theabove-mentioned processing of step S106.

In this manner, by carrying out the processing of step S109 by means ofthe ECU 20, an “adjustment unit” according to the present invention isachieved.

Moreover, in cases where a negative determination is made in theabove-mentioned processing of step S105 (Tcat>Tlow), the routine of theECU 20 goes to the processing of step S110, where the throttle valve 71is controlled in such a manner that the amount of intake air in theinternal combustion engine 1 becomes equal to the temperature risesuppression amount which is larger than the cooling suppression amount.

When the amount of intake air during the fuel cut-off operation (theamount of air passing through the exhaust gas purification device 81) iscontrolled by the above-mentioned procedure, even in cases where thefuel cut-off operation of the internal combustion engine 1 is carriedout in the state where the temperature of the catalyst is low, thetemperature of the catalyst becomes difficult to drop to less than theactivation temperature thereof, at the time of the low-load operation ofthe internal combustion engine during the fuel cut-off operation orafter the end of the fuel cut-off operation. As a result, the catalystcan be maintained in its activated state, thus making it possible tosuppress the deterioration of exhaust emissions after the end of thefuel cut-off operation.

Here, note that due to the effect of the amount of intake air beingadjusted to the reaction promotion amount, the temperature of thecatalyst may become higher than the low-temperature determinationtemperature in the course of the rich period of time. In such a case,when the amount of intake air in the internal combustion engine 1 iscontrolled according to the processing routine of FIG. 4, the amount ofintake air in the internal combustion engine 1 is changed from thereaction promotion amount to the temperature rise suppression amount inthe course of the rich period of time. Moreover, when the temperature ofthe catalyst drops to a temperature equal to or less than thelow-temperature determination temperature in the subsequent rich periodof time, the amount of intake air in the internal combustion engine 1 isagain set to the temperature rise suppression amount.

Here, in the arrangement in which the amount of air passing through theexhaust gas purification device 81 is changed by changing the degree ofopening of the throttle valve 71, it takes some time until the change inthe degree of opening of the throttle valve 71 is reflected on theamount of air passing through the exhaust gas purification device 81. Onthe other hand, in cases where the fuel cut-off operation of theinternal combustion engine 1 is carried out, the rich period of time inwhich the interior of the exhaust gas purification device 81 becomes arich atmosphere is relatively short. Accordingly, even if the degree ofopening of the throttle valve 71 is changed during the rich period oftime, the rich period of time may end before the change in the degree ofopening of the throttle valve 71 is reflected on the amount of airpassing through the exhaust gas purification device 81.

Accordingly, in the arrangement in which the amount of air passingthrough the exhaust gas purification device 81 is adjusted by changingthe degree of opening of the throttle valve 71 at the time of the fuelcut-off operation, when the catalyst temperature at the start time pointof the fuel cut-off operation is equal to or higher than the activationtemperature, and at the same time is equal to or lower than thelow-temperature determination temperature, the amount of intake airduring the rich period of time may be fixed to the reaction promotionamount.

However, in cases where the amount of air passing through the exhaustgas purification device 81 during the fuel cut-off operation is adjustedby making use of a variable valve operating mechanism, a response delayis smaller than in the case of using the throttle valve 71, and so theamount of air passing through the exhaust gas purification device 81 maybe adjusted according to the processing routine of FIG. 4.

Further, in cases where the amount of air passing through the exhaustgas purification device 81 during the fuel cut-off operation is adjustedby making use of a secondary air supply device, too, a response delay issmaller than in the case of using the throttle valve 71, and hence, theamount of air passing through the exhaust gas purification device 81 mayalso be adjusted according to the processing routine of FIG. 4. Here,note that in the case of using the secondary air supply device, thethrottle valve 71 may be controlled in such a manner that the amount ofintake air during the period of time of the fuel cut-off operationbecomes equal to the cooling suppression amount, whereas when it isnecessary to adjust the amount of air passing through the exhaust gaspurification device 81 to the temperature rise suppression amount,secondary air may be applied from the secondary air supply device to theexhaust passage 80 upstream of the exhaust gas purification device 81.

Embodiment 2

Next, reference will be made to a second embodiment of the presentinvention based on FIGS. 5 and 6. Here, a construction different fromthat of the above-mentioned first embodiment will be described, and anexplanation of the same construction will be omitted.

In the above-mentioned first embodiment, there has been described anexample in which when the temperature of the catalyst is higher than thelow-temperature determination temperature in the period of time of thefuel cut-off operation, the amount of air passing through the exhaustgas purification device 81 is made equal to the temperature risesuppression amount. In contrast to this, in this second embodiment,there will be described an example in which in the rich period of timeimmediately after the start of the fuel cut-off operation, when thecatalyst temperature is equal to or higher than a predeterminedhigh-temperature determination temperature which is higher than thelow-temperature determination temperature, the amount of air passingthrough the exhaust gas purification device 81 is adjusted to an amountsmaller than the temperature rise suppression amount.

As described in the above-mentioned first embodiment, when the catalysthas been activated in the rich period of time immediately after thestart of the fuel cut-off operation, oxidation reaction is caused tooccur by the catalyst, so that the catalyst is heated by the heat of thereaction. At that time, when the temperature of the catalyst in the richperiod of time is very high as compared with the low-temperaturedetermination temperature, the catalyst may be caused to rise intemperature to an excessive extent by the heat of the oxidationreaction. In particular, when the amount of air passing through theexhaust gas purification device 81 is made equal to the temperature risesuppression amount during the rich period of time, a relatively largeamount of oxidation reaction heat is generated, thus making it easy forthe catalyst to rise in temperature to an excessive extent.

In contrast to this, in this second embodiment, in cases where the fuelcut-off operation of the internal combustion engine 1 is carried out ina state where the temperature of the catalyst received in the exhaustgas purification device 81 becomes equal to or higher than thepredetermined high-temperature determination temperature, the throttlevalve 71 is controlled in the rich period of time immediately after thestart of the fuel cut-off operation (i.e., a period of time from t1 tot2 in FIG. 5) in such a manner that the amount of intake air in theinternal combustion engine 1 becomes equal to a predetermined reactionsuppression amount which is smaller than the amount of intake air afterthe lapse of the rich period of time (the temperature rise suppressionamount), as shown in FIG. 5. Then, in a period of time from the end timepoint (t2 in FIG. 5) of the rich period of time until the F/C flag isturned off, the throttle valve 71 is controlled so as to make the amountof intake air in the internal combustion engine 1 equal to thetemperature rise suppression amount. The “high-temperature determinationtemperature” referred to herein is a temperature which is consideredthat when the amount of intake air in the rich period of time is madeequal to the temperature rise suppression amount in the state where thetemperature of the catalyst is higher than the high-temperaturedetermination temperature, the temperature of the catalyst rises to anexcessive extent during the rich period of time. This high-temperaturedetermination temperature is a temperature (e.g., about from 700 to 800degrees C.) which has been obtained experimentally in advance.

Here, note that the rate of the oxidation reaction occurring in thecatalyst during the rich period of time becomes slower (smaller) inaccordance with the decreasing amount of oxygen supplied to the catalystper unit time. For that reason, it is preferable that the amount ofintake air during the rich period of time be as small as possible.Accordingly, the “reaction suppression amount” referred to herein is anamount which is smaller than the temperature rise suppression amount,and is desirable to be set to the above-mentioned lower limit value (theminimum amount of intake air in which the amount of oil sucked into acylinder 2 due to an increase of negative pressure in the cylinder 2accompanying a decrease in the amount of intake air falls below thepermissible amount), or an amount equal to a sum of the lower limitvalue and a predetermined margin.

In the case where the amount of air passing through the exhaust gaspurification device 81 is made equal to the reaction suppression amountduring the rich period of time, the amount of oxygen flowing into theexhaust gas purification device 81 per unit time becomes smaller, incomparison with the case where the amount of air passing through theexhaust gas purification device 81 is made equal to the temperature risesuppression amount. As the amount of oxygen flowing into the exhaust gaspurification device 81 per unit time decreases, the amount of oxidationreaction heat generated per unit time also decreases. As a result, thecarrying away of heat by the air passing through the catalyst becomesprevalent with respect to the heating of the catalyst by the heat ofoxidation reaction, so that the amount of rise in the temperature of thecatalyst during the rich period of time decreases. Consequently, thecatalyst becomes difficult to rise excessively in temperature during therich period of time.

Hereinbelow, reference will be made to a control procedure for theamount of intake air in the case where the fuel cut-off operation of theinternal combustion engine 1 is carried out, in line with FIG. 6. FIG. 6is a flow chart showing a processing routine which is carried outrepeatedly by the ECU 20 during the operation of the internal combustionengine 1. In the processing routine of FIG. 6, the same symbols areattached to the same processes as those in the above-mentionedprocessing routine of FIG. 4. Here, it is assumed that Thigh in FIG. 6indicates the high-temperature determination temperature.

The difference of the processing routine in FIG. 6 from the processingroutine in FIG. 4 is in the processings which are carried out, in caseswhere a negative determination is made in the processing of step S105(Tcat>Tlow). Accordingly, in the following, among the processings of theprocessing routine in FIG. 6, those which are different from theabove-mentioned processings in FIG. 4 will be described, and anexplanation of the same processings will be omitted.

In cases where an affirmative determination is made in the processing ofstep S105, the routine of the ECU 20 goes to the processing of stepS201, where it is determined whether the temperature Tcat of thecatalyst obtained in the processing of step S103 is equal to or higherthan the predetermined high-temperature determination temperature Thigh.In cases where a negative determination is made in the processing ofstep S201 (Tcat<Thigh), the routine of the ECU 20 goes to the processingof step S110, where the throttle valve 71 is controlled in such a mannerthat the amount of intake air in the internal combustion engine 1becomes equal to the temperature rise suppression amount.

On the other hand, in cases where an affirmative determination is madein the above-mentioned processing of step S201 (Tcat≥Thigh), the routineof the ECU 20 goes to the processing of step S202. In the processing ofstep S202, the ECU 20 determines whether the air fuel ratio AFcatobtained in the processing of step S104 is equal to or lower than thestoichiometric air fuel ratio AFst.

In cases where an affirmative determination is made in theabove-mentioned processing of step S202 (AFcat≤AFst), the current pointin time belongs to the above-mentioned rich period of time in FIG. 5.Accordingly, the routine of the ECU 20 goes to the processing of stepS203, where the throttle valve 71 is controlled in such a manner thatthe amount of intake air in the internal combustion engine 1 becomesequal to the reaction suppression amount which is smaller than thetemperature rise suppression amount.

On the other hand, in cases where a negative determination is made inthe above-mentioned processing of step S202 (AFcat>AFst), at the currentpoint in time, the above-mentioned rich period of time has alreadypassed. Accordingly, the routine of the ECU 20 goes to the processing ofstep S110, where the throttle valve 71 is controlled in such a mannerthat the amount of intake air in the internal combustion engine 1becomes equal to the temperature rise suppression amount.

When the amount of intake air during the fuel cut-off operation (theamount of air passing through the exhaust gas purification device 81) iscontrolled by the above-mentioned procedure, in cases where the fuelcut-off operation of the internal combustion engine 1 is carried out ina state where the temperature of the catalyst is low, the same effectsor advantages as those in the above-mentioned first embodiment can beobtained. In addition, in cases where the fuel cut-off operation of theinternal combustion engine is carried out in a state where thetemperature of the catalyst is very high, the catalyst becomes difficultto rise excessively in temperature during the rich period of time, sothat the thermal deterioration of the catalyst can be suppressed.

Here, note that due to the effect of the amount of intake air beingadjusted to the reaction suppression amount, the temperature of thecatalyst may drop to less than the high-temperature determinationtemperature in the course of the rich period of time. In such a case,when the amount of intake air in the internal combustion engine 1 iscontrolled according to the processing routine of FIG. 6, the amount ofintake air in the internal combustion engine 1 is changed from thereaction suppression amount to the temperature rise suppression amountin the course of the rich period of time. Moreover, when the temperatureof the catalyst rises to a temperature equal to or higher than thehigh-temperature determination temperature in the subsequent rich periodof time, the amount of intake air in the internal combustion engine 1 isagain changed to the reaction suppression amount.

Here, as described in the above-mentioned first embodiment, in thearrangement in which the amount of air passing through the exhaust gaspurification device 81 is changed by changing the degree of opening ofthe throttle valve 71, even if the degree of opening of the throttlevalve 71 is changed during the rich period of time, the rich period oftime may end before the change in the degree of opening of the throttlevalve 71 is reflected on the amount of air passing through the exhaustgas purification device 81.

Accordingly, in the arrangement in which the amount of air passingthrough the exhaust gas purification device 81 is adjusted by changingthe degree of opening of the throttle valve 71 at the time of the fuelcut-off operation, when the catalyst temperature at the start time pointof the fuel cut-off operation is equal to or higher than thehigh-temperature determination temperature, the amount of intake airduring the rich period of time may be fixed to the reaction suppressionamount.

However, in cases where the amount of air passing through the exhaustgas purification device 81 during the fuel cut-off operation is adjustedby making use of a variable valve operating mechanism, a response delayis smaller than in the case of using the throttle valve 71, and so theamount of air passing through the exhaust gas purification device 81 maybe adjusted according to the processing routine of FIG. 6.

Further, in cases where the amount of air passing through the exhaustgas purification device 81 during the fuel cut-off operation is adjustedby making use of a secondary air supply device, too, a response delay issmaller than in the case of using the throttle valve 71, and hence, theamount of air passing through the exhaust gas purification device 81 mayalso be adjusted according to the processing routine of FIG. 6.

REFERENCE SIGNS LIST

-   1 internal combustion engine-   2 cylinder-   5 fuel injection valve-   6 spark plug-   7 intake port-   8 exhaust port-   11 water temperature sensor-   20 ECU-   70 intake passage-   71 throttle valve-   72 air flow meter-   80 exhaust passage-   81 exhaust gas purification device-   82 exhaust gas temperature sensor-   83 air fuel ratio sensor

What is claimed is:
 1. A control apparatus for an internal combustionengine in which in an exhaust passage, there is disposed a catalysthaving an oxygen storage ability in which oxygen is stored when an airfuel ratio of exhaust gas is a lean air fuel ratio higher than astoichiometric air fuel ratio, whereas oxygen is released when the airfuel ratio of exhaust gas is a rich air fuel ratio lower than thestoichiometric air fuel ratio, the control apparatus comprising: acontroller comprising at least one processor configured to: estimate theair fuel ratio of the exhaust gas in said catalyst; obtain a temperatureof said catalyst; adjust an amount of air passing through said catalystto a predetermined cooling suppression amount that minimizes cooling ofthe catalyst, when the obtained temperature is equal to or less than apredetermined low-temperature determination temperature which is higherthan an activation temperature of said catalyst, in a period of time inwhich the internal combustion engine is driven in a fuel cut-offoperation; and change the amount of air passing through said catalyst tomore than said cooling suppression amount, when the obtained temperatureis equal to or higher than the activation temperature of said catalyst,during a period of time from a point in time at which the fuel cut-offoperation is started until a point in time at which the estimated airfuel ratio changes from an air fuel ratio equal to or less than thestoichiometric air fuel ratio to a lean air fuel ratio, in a period oftime in which the amount of air passing through said catalyst isadjusted to the cooling suppression amount, wherein the controlleradjusts the amount of air passing through the catalyst to apredetermined temperature rise suppression amount which is larger thanthe cooling suppression amount, when the obtained temperature is equalto or higher than a predetermined high-temperature determinationtemperature which is higher than the low-temperature determinationtemperature, in the period of time in which the internal combustionengine is driven in the fuel cut-off operation, and decreases the amountof air passing through the catalyst from the temperature risesuppression amount, when the obtained temperature is equal to or higherthan the predetermined high-temperature determination temperature whichis higher than the low-temperature determination temperature, during theperiod of time from the point in time at which the fuel cut-offoperation is started until the point in time at which the estimated airfuel ratio changes from the air fuel ratio equal to or less than thestoichiometric air fuel ratio to the lean air fuel ratio, in the casewhere the amount of air passing through the catalyst is adjusted to thetemperature rise suppression amount.
 2. The control apparatus for aninternal combustion engine as set forth in claim 1, the controlapparatus further comprising: a measuring device that is disposed in theexhaust passage at a location downstream of the catalyst for measuring aphysical quantity correlated with the air fuel ratio of the exhaust gas,wherein the controller estimates the air fuel ratio of the exhaust gasinside the catalyst, by assuming that the air fuel ratio measured by themeasuring device is equal to the air fuel ratio of the exhaust gasinside the catalyst.
 3. A control apparatus for an internal combustionengine in which in an exhaust passage, there is disposed a catalysthaving an oxygen storage ability in which oxygen is stored when an airfuel ratio of exhaust gas is a lean air fuel ratio higher than astoichiometric air fuel ratio, whereas oxygen is released when the airfuel ratio of exhaust gas is a rich air fuel ratio lower than thestoichiometric air fuel ratio, comprising: an air-fuel ratio sensorconfigured to estimate the air fuel ratio of the exhaust gas in saidcatalyst; a temperature sensor configured to obtain a temperature ofsaid catalyst; a throttle valve disposed in an intake passage; acontroller comprising at least one processor configured to control thethrottle valve to: adjust an amount of air passing through said catalystto a predetermined cooling suppression amount that minimizes cooling ofthe catalyst, when the obtained temperature is equal to or less than apredetermined low-temperature determination temperature which is higherthan an activation temperature of said catalyst, in a period of time inwhich the internal combustion engine is driven in a fuel cut-offoperation; and change the amount of air passing through said catalyst tomore than said cooling suppression amount, when the obtained temperatureis equal to or higher than the activation temperature of said catalyst,during a period of time from a point in time at which the fuel cut-offoperation is started until a point in time at which the estimated airfuel ratio changes from an air fuel ratio equal to or less than thestoichiometric air fuel ratio to a lean air fuel ratio, in a period oftime in which the amount of air passing through said catalyst isadjusted to the cooling suppression amount, wherein the controlleradjusts the amount of air passing through the catalyst to apredetermined temperature rise suppression amount which is larger thanthe cooling suppression amount, when the obtained temperature is equalto or higher than a predetermined high-temperature determinationtemperature which is higher than the low-temperature determinationtemperature, in the period of time in which the internal combustionengine is driven in the fuel cut-off operation, and decreases the amountof air passing through the catalyst from the temperature risesuppression amount, when the obtained temperature is equal to or higherthan the predetermined high-temperature determination temperature whichis higher than the low-temperature determination temperature, during theperiod of time from the point in time at which the fuel cut-offoperation is started until the point in time at which the estimated airfuel ratio changes from the air fuel ratio equal to or less than thestoichiometric air fuel ratio to the lean air fuel ratio, in the casewhere the amount of air passing through the catalyst is adjusted to thetemperature rise suppression amount.